This invention relates to an SF.sub.6 /nitriding gas plasma etching or reactive ion etching system. More particularly, the invention relates to an improvement in an etchant system of the kind disclosed in U.S. Pat. application Ser. No. 522,437 filed Aug. 11, 1983, U.S. Pat. No. 4,465,552, relating to selectively etching silicon dioxide with an SF.sub.6 /nitriding component gas composition. The present invention affords improved selectivity in etching systems of this kind.
An important step in the manufacture of semiconductor chips and thin film circuitry is the etching of the different layers such as polysilicon and silicon dioxide which make up the finished semiconductor chip or the thin film circuit. In the manufacture of these thin film circuits, one method of etching has been to overlay the surface to be etched with a suitable mask and to immerse the circuit so masked in a chemical solution which attacks the surface to be etched while leaving the mask otherwise substantially intact. It has been difficult with the chemical etching processes presently known to achieve at all times well-defined edges on the etched surfaces. The difficulty arises because the chemical action tends to etch isotropically, i.e., the chemical etching undercuts the mask by the same distance that it penetrates the underlayer, and thus enlarges the feature. It is, therefore, very difficult to use wet chemical etching to achieve fine structures of good sharp quality; fine structures being defined as structures having geometries on the order of one micron.
One known method for manufacturing semiconductor chips is called "plasma etching" in which a vacuum container is filled with a low pressure gas such as a fluorocarbon or SF.sub.6. A surface to be etched is covered by a mask and inserted into the container along with the reactive gas. To etch the surface, an ac voltage whose frequency may be between 20 kHz and 30 Ghz is applied to excite the fluorocarbon or SF.sub.6, thereby dissociating the fluorocarbon or SF.sub.6 and forming various positive and negative ions, reactive neutral species (e.g., F atoms), and electrons. The dissociated species interact with the surface to be etched producing various gases as reaction products.
The ability to etch thin (approximately 1 .mu.) layers of SiO.sub.2 over Si is an important aspect of the plasma or reactive ion etch processes used in microcircuit manufacture. The plasma field of any given apparatus or oxide layer thickness is not necessarily uniform over the entire wafer surface, thus the etch may penetrate the SiO.sub.2 layer at different times. In order to avoid etching the underlying silicon underlayer where the oxide has first cleared, it is desirable to arrange a selective plasma process which does not seriously affect or diminish the etch rate on SiO.sub.2 but significantly decreases the rate on silicon.
We have found in working with the SF.sub.6 /nitriding plasma composition of the above noted U.S. Pat. application Ser. No. 522,437, particularly when using a low frequency excitation source, that selectivity can be significantly improved when the SF.sub.6 /nitriding plasma is modified with an oxidizing component. The improvement afforded by using the combination including an oxidizing component substantially enhances the practical processing of the work pieces to be etched.